This invention relates to a system and method for mobile stations to maintain multiple physical channels to basestations within a direct sequence-code division multiple access mobile communications system and more specifically, to the handoff operation of such mobile stations and basestations.
Improvements are consistently being made on previous cellular technology standards. Each subsequent standard has benefits over the previous, such as increased bandwidth, reductions in interference, or simply more flexible functionality. Since mobile stations that function within these standards are portable, one key component of all standards is the method by which an active call or session can be transferred between basestations, hereinafter referred to as handoff. Such a procedure switches the current radio link for a mobile station from a first basestation to a second basestation in order to maintain communications when the mobile station experiences changes in the communication conditions. Changes in communication conditions can be caused by crossing boundaries of basestation cells, falling into deep fades, or a rearrangement of radio resources at the basestation level. Adjustments in the techniques used to handoff active sessions between basestations must be made in parallel with fundamental changes to the overall standard.
Direct Spreading (DS)-Code Division Multiple Access (CDMA) technology standards are going through an ongoing process of improvements and updates. The xe2x80x9coriginalxe2x80x9d DS-CDMA standard is a second generation CDMA standard defined in Telecommunications Industry Association/Electronic Industry Association/Interim Standard 95-A (TIA/EIA/IS-95-A). This cellular standard operates predominantly for voice services in full duplex mode. For every forward link (FL) CDMA channel that is established, a reverse link (RL) CDMA channel is also initiated. This standard does not support asymmetrical service to be provided to the user.
The handoff between basestations in the IS-95-A standard can be triggered in two ways. First, it can be triggered by the mobile station due to a deterioration in the signal strength or quality at the mobile station. Secondly, it can be triggered by one of the basestations currently communicating with the mobile station, hereinafter referred to as active basestations, due to a deterioration in the signal strength or quality at the basestation or due to bandwidth constraints at the basestation which require the handoff of an active session to another basestation, hereinafter referred to as a target basestation.
The IS-95-A standard supports soft, softer, and hard handoff for voice services from an active basestation to a target basestation. In soft handoff, both the active and target basestations have a forward and reverse CDMA channel established with the mobile station prior to the termination of the forward and reverse CDMA channels between the mobile station and the active basestation and prior to the target basestation""s assumption of the role of active basestation. Softer handoff is identical to soft handoff, but the xe2x80x9cactivexe2x80x9d and xe2x80x9ctargetxe2x80x9d basestations are actually resources provided by the same basestation. In hard handoff, the target basestation replaces the active basestation by immediately terminating the forward and reverse CDMA channels between the mobile station and the active basestation before the new channels are fully established.
Handoffs triggered by the mobile station are performed on the basis of signal strengths measured by each mobile station for basestations which are in range of the mobile station. A special channel called the xe2x80x9cpilot channelxe2x80x9d is generated by each basestation for the purpose of this measurement. Certain basestations that are equipped with multiple directional antennae are capable of functioning with multiple pilot channels, each pilot channel corresponding to a particular sector. Basestations capable of operating in multiple frequency bands may generate a pilot channel for each such band.
Within the IS-95-A standard, there are four pilot sets of identifiers defined for use in the normal operation of a mobile station. Each of these sets of identifiers contains information which identifies or permits an identification of a set of basestations resources. For example, each pilot channel includes a code which identifies basestation resources consisting of a basestation and/or a sector of a basestation and/or a frequency band and these codes can be used in the sets of identifiers. These sets include an Active Set (AS), a Candidate Set (CS), a Neighbour Set (NS), and a Remaining Set (RS). The AS contains the set of identifiers which correspond with basestation resources currently assigned to the particular mobile station. The CS contains the set of identifiers for basestations transmitting pilot channels with sufficient signal strength to be successfully demodulated, but that have not been placed in the AS. The NS contains the set of identifiers of basestation resources that are probable candidates for handoff, normally due to the fact that they are geographically near the mobile station. Finally, the RS contains the set of all identifiers for allowable basestation resources within the current frequency assignment excluding those included in the AS, CS, and NS.
In simplistic terms, the handoff procedure is done with the updating of the AS which changes the set of basestation resources assigned to the particular mobile station. Whenever a change in the AS occurs, both the mobile station and the relevant basestations must take the necessary steps to set up or tear down channels such that only basestations which exist in the updated AS have active channels.
There are a number of issues concerning the IS-95-A standard that resulted in another second generation CDMA standard being formed. One of these issues, as discussed previously, is that the IS-95-A standard does not support the use of asymmetrical communications. The enhanced second generation CDMA standard, defined by TIA/EIA/IS-95-B, continues assigning a fundamental channel (FCH) with both a FL CDMA channel and a RL CDMA channel to be used for voice service, but also allows for the assignment of FL and RL supplemental channels (SCHs) separately for use in limited data services. These SCHs allow communication systems defined by the IS-95-B standard to handle asymmetrical data communications in a more efficient method than previously done with the IS-95-A standard.
There are numerous other advances made in the IS-95-B CDMA standard that make it significantly more flexible compared to the IS-95-A standard. For example, the initial AS during the initialization of an active session for the IS-95-B standard is capable of containing multiple pilot channels, thus allowing a session to be started between the mobile station and more than one basestation. As well, there is a significant enhancement with regard to inter-frequency handoff procedures within the IS-95-B standard that was not addressed in the IS-95-A standard. In the IS-95-B standard, a basestation can direct a mobile station to search for pilot channels in different operating frequencies through the temporary termination of normal communications. With use of pilot channel measurement reports sent from the mobile station, the basestation makes inter-frequency handoff decisions. A Candidate Frequency Neighbor Set (CFNS) and a Candidate Frequency Search Set (CFSS) are added to the previously defined AS, CS, NS, and RS in the IS-95-B standard to enable the inter-frequency handoff capability. The CFNS is a set of all pilot channels available on the candidate frequency while the CFSS is a subset of the CFNS that includes all pilot channels on the candidate frequency that are to be searched for suitable signal strength.
Overall, handoff in the IS-95-B standard, defined within the TIA/EIA/SP-3693 standard pages 6-383 to 6-462 and pages 7-167 to 7-181, hereby incorporated by reference, is performed in a similar fashion to that done for the IS-95-A standard, but with additional options and flexibility. There are backup procedures, in case of failure during hard handoff, within the IS-95-B standard that are not defined within the IS-95-A standard. In the situation of a failure to initiate communications with the target basestation, the mobile station re-establishes a channel with the original basestation. Another difference between the two standards is the previously mentioned inter-frequency handoff capability that requires modified handoff management signals.
Similar to the IS-95-A standard handoff for the FL and RL CDMA channels, the IS-95-B standard requires the FCHs and SCHs to be transferred simultaneously with use of the same type of handoff, soft or hard. Therefore, during soft handoff, all of the established physical channels must be supported by both the first and second basestations. If it is not possible for all physical channels to perform a soft handoff, a hard handoff is likely performed for all of the channels.
The third generation DS-CDMA standard, referred to as the 3G DS-CDMA standard, is currently defined with increased flexibility and bandwidth than the IS-95-B standard. Further improvements are suggested in a preliminary report entitled xe2x80x9cThe cdma2000 RTT Candidate Submissionxe2x80x9d published Jun. 2, 1998 by the Technical Reference 45.5 wideband spread spectrum digital technology subcommittee of the mobile and personal communications public 800 standards committee of the TIA for submission to the U.S. Task Group 8/1 of the International Telecommunication Union, hereby incorporated by reference. The 3G DS-CDMA systems have a different physical layer design and support many more services than either IS-95-A or IS-95-B. These systems are specifically designed to handle voice service, circuit data service, packet data service, or any combination of such services. A voice service requires a FCH with a FL and a RL and possibly an additional physical channel for signalling. A circuit data service or a packet data service requires one or more SCHs assigned on the FL and/or the RL, depending upon the packet data information flow directions, and possibly requires a dedicated control channel (DCCH) established in both directions to carry power control, radio link protocol (RLP), media access control (MAC) and/or layer 2/3 signalling. The 3G DS-CDMA standard allows for the establishment of a FCH with a FL and a RL, one or more FL-SCHs and/or RL-SCHs, and a DCCH with a FL and a RL. Therefore, systems using the 3G DS-CDMA standard are capable of operating voice, circuit data, and packet data services simultaneously. An additional feature of the 3G DS-CDMA standard is the capability of having SCHs shared between packet data users. This is efficient for cases in which a packet data service is bursty with individual users only transmitting occasionally.
One key problem that needs to be addressed with regard to the 3G DS-CDMA standard is a how to perform a handoff that can support the extensive flexibility of the standard and cope with the 3G physical layer design. The use of the IS-95-B handoff procedure within systems supporting the 3G DS-CDMA standard would result in handoff scenarios with high costs on efficiency and flexibility and handoff scenarios not being supported at all, causing a high call blocking rate and/or interference level.
A handoff technique proposed for the 3G DS-CDMA standard within the above-referenced preliminary report continues use of the four pilot sets defined for the IS-95-A standard, those being the AS, CS, NS, and RS. Since the preliminary report requires reverse compatibility with IS-95-A and IS-95-B, it is therefore assumed that the previously defined pilot sets, CFNS and CFSS, from the IS-95-B standard are also maintained. In the proposed handoff, the AS is divided into a Full Active Set (FAS) and a Reduced Active Set (RAS). The RAS is a subset of the larger FAS and is used in situations where specific channels do not have similar handoff capabilities or do not require the same handoff arrangement as other channels. The 3G DS-CDMA standard, as currently defined, has not established a procedure or an implementation for such a handoff with the FAS and the RAS.
There are limitations on the handoff as contemplated by the cdma2000 proposal. For one, the defined RAS, according to the cdma2000 proposal, is used solely for physical channels carrying unconstrained delay data applications. Hence, the standard still requires the handoff of most physical channels to be done in parallel and simultaneously with use of the FAS, which is comparable to the IS-95-B standard.
A handoff procedure is thus needed for the 3G DS-CDMA standard that can utilize the full flexibility incorporated within the 3G standard.
It is an object of the present invention to mitigate one or more disadvantages of the prior art and, in particular, to provide a technique for handing off an active session between a mobile station and at least one basestation with the flexibility of transferring specific channels and/or channel types at the same or different times and using the same or different handoff methods.
The present invention, according to a broad aspect, provides a mobile station that has a number physical channels between itself and a set of basestations. Groups of one or more physical channels are defined, for example according to channel type. The mobile station is capable of performing handoffs independently for each of the groups of physical channels. In an exemplary embodiment, a plurality of groups have associated active sets that when updated allow the handoffs for the groups of channels associated with the particular active sets to be performed independently.
In further aspects, the present invention provides a basestation, a mobile communication system, and a method of performing handoffs. In each of these aspects, a mobile station is arranged to perform independent handoffs for groups of physical channels between the mobile station and a set of basestations similar to that in the broad aspect described above.
Preferably, the invention is applied in the context of a 3G DS-CDMA system in which the physical channels between the mobile station and the set of basestations consist of a fundamental channel, forward and reverse link supplemental channels, and a dedicated control channel. In this preferred embodiment, separate active sets are maintained for each of these channels.